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Molecular ordering at electrified interfaces: Template andpotential effectsThanh Hai Phan*1,2,3 and Klaus Wandelt1,4

Full Research Paper Open Access

Address:1Institute of Physical and Theoretical Chemistry, University of Bonn,Wegelerstr. 12, 53115 Bonn, Germany, 2Laboratory ofPhotochemistry and Spectroscopy, Department of Chemistry, CatholicUniversity of Leuven, Celestijnenlaan 200F, B-3001, Hevelee,Belgium, 3Physics Department, Quynhon University, 170 An DuongVuong; Quynhon, Vietnam and 4Institute of Experimental Physics,University of Wroclaw, MaxaBorna 9, 50-204, Wroclaw, Poland

Email:Thanh Hai Phan* - phan@thch.uni-bonn.de

* Corresponding author

Keywords:cyclic voltammogram; scanning tunneling microscopy; self-assembly;template effect; viologen

Beilstein J. Org. Chem. 2014, 10, 2243–2254.doi:10.3762/bjoc.10.233

Received: 26 May 2014Accepted: 03 September 2014Published: 23 September 2014

This article is part of the Thematic Series "Chemical templates".

Guest Editor: S. Höger

© 2014 Phan and Wandelt; licensee Beilstein-Institut.License and terms: see end of document.

AbstractA combination of cyclic voltammetry and in situ scanning tunneling microscopy was employed to examine the adsorption and

phase transition of 1,1’-dibenzyl-4,4’-bipyridinium molecules (abbreviated as DBV2+) on a chloride-modified Cu(111) electrode

surface. The cyclic voltammogram (CV) of the Cu(111) electrode exposed to a mixture of 10 mM HCl and 0.1 mM DBVCl2 shows

three distinguishable pairs of current waves P1/P’1, P2/P’2, and P3/P’3 which are assigned to two reversible electron transfer steps,

representing the reduction of the dicationic DBV2+ to the corresponding radical monocationic DBV+• (P1/P’1) and then to the

uncharged DBV0 (P3/P’3) species, respectively, as well as the chloride desorption/readsorption processes (P2/P’2). At positive

potentials (i.e., above P1) the DBV2+ molecules spontaneously adsorb and form a highly ordered phase on the c(p × √3)-precov-

ered Cl/Cu(111) electrode surface. A key element of this DBV2+ adlayer is an assembly of two individual DBV2+ species which,

lined up, forms a so-called “herring-bone” structure. Upon lowering the electrode potential the first electron transfer step (at P1)

causes a phase transition from the DBV2+-related herring-bone phase to the so-called "alternating stripe" pattern built up by the

DBV+• species following a nucleation and growth mechanism. Comparison of both observed structures with those found earlier at

different electrode potentials on a c(2 × 2)Cl-precovered Cu(100) electrode surface enables a clear assessment of the relative impor-

tance of adsorbate–substrate and adsorbate–adsorbate interactions, i.e., template vs self-assembly effects, in the structure formation

process of DBV cations on these modified Cu electrode surfaces.

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Beilstein J. Org. Chem. 2014, 10, 2243–2254.

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IntroductionThe precise control of the self-organization of molecular layers

on either conducting or dielectric substrates is regarded as one

of the key factors in the successful design, characterization and

fabrication of nanoscale molecular devices [1-4]. A big chal-

lenge for surface scientists is, thus, to find suitable model

systems which enable to investigate the driving forces of molec-

ular self-organization on surfaces and to simulate the working

principles of the derived molecular devices. This so-called

“bottom-up” strategy, i.e., the formation of supramolecular

structures from vapor deposited simpler building blocks has

become an important research direction in ultra-high vacuum

(UHV) based surface science in recent years. However,

promising organic compounds may not remain intact volatile,

and may thus not be deposited via the gas phase. In those cases,

it may be a promising strategy to deposit these molecules from

solution instead. Besides, this preparation route is probably also

more economic than operating a vacuum evaporation system, in

particular if the organic building blocks come in water-soluble

form.

In principle, the self-organization process of molecular adsor-

bates is driven by the interplay between adsorbate–adsorbate

and adsorbate–substrate interactions. While the former depend

on the specific nature of the molecular building blocks, e.g.,

their shape, polarity, functional groups, etc., and may include all

possible interactions from van der Waals forces to covalent

bonds, the latter are described by the so-called “corrugation

function”, i.e., the two-dimensional (2D) potential energy land-

scape representing the minimum total energy for all possible

adsorbate–substrate configurations and, thereby, the interaction

strength between the substrate and an adsorbate molecule at any

surface site. If, in equilibrium, the adsorbate–adsorbate interac-

tions dominate over the adsorbate–substrate interactions the

molecules will essentially “self-assemble” independent of the

substrate surface. If, however, the adsorbate–substrate interac-

tions are very strong compared to the intermolecular forces the

substrate will influence the structure of the adsorbate layer,

provided a sufficient surface mobility allows the adsorbed

molecules to reach their equilibrium positions, i.e., minima in

the corrugation function. In this case the substrate surface acts

as a “template”. Under UHV conditions the activation energy

for structural equilibration is usually provided by heating the

substrate. If however, the organic species are deposited in ionic

form from aqueous solution, as done in the present work, the

obtained structure will additionally be influenced by electro-

static forces acting between the molecules and the substrate as

well as between the molecules themselves. In this respect elec-

trochemical deposition has the additional advantage that these

electrostatic interactions can be “tuned” by the electrochemical

potential in two ways. On the one hand the mere charge density

at the electrode surface itself determines the electrostatic forces

between adsorbed ions, not only the organic species but also

possibly co-adsorbed other ions present in the solution, and the

substrate. On the other hand, driven by the electrode potential

the molecular ions may undergo redox-reactions, thereby

changing their own charge state. Both cases are expected to

influence the deposition and structure formation of the molec-

ular layers.

In this paper we will present results on the self-organization of

1,1’-dibenzyl-4,4’-bipyridinium, in short dibenzyl-viologen

(DBV), cations on a chloride precovered Cu(111) electrode

surface. A comparison of these findings with those described

earlier for the same molecules on a chloride-modified Cu(100)

electrode [5-7], will then enable us to arrive at a generalized

picture of the influence of template and potential effects on the

structure formation of these molecular ions on both chloride

modified copper single crystal surfaces of different symmetry.

The motivation for the choice of viologen molecules is twofold.

On the one hand molecular viologen-based self-assemblies have

attracted a great deal of attention in recent years due to their

widespread applications in electronic devices [8,9], and light-

harvesting operators [10]. On the other hand the electrochem-

istry of viologens in solution is well documented in the litera-

ture [11,12]. In dicationic form dibenzyl-viologen molecules

(DBV2+) are well-known to undergo two successive reversible

electron transfer steps yielding first the corresponding monoca-

tion radical DBV+• and then the uncharged viologen species

DBV0, respectively. The first investigations on the surface

redox chemistry as well as the self-assembly of DBV-species on

a chloride modified Cu(100) surface, were presented by

Safarowsky et al. and Pham et al. [5-7]. In the present paper we

will describe for the first time the structural properties of self-

assembled DBV on a chloride terminated Cu(111) electrode

surface. Their comparison with the previous results obtained on

the chloride precovered Cu(100) surface will clearly demon-

strate the relative importance of adsorbate–substrate and adsor-

bate–adsorbate interactions, i.e., template-effects vs self-

assembly, at different electrode potentials.

ResultsElectrochemical characterizationThe electrochemical characterization of the Cu(111) surface in

both pure 10 mM HCl and the viologen containing (10 mM HCl

+ 0.1 mM DBV2+) solution was done using cyclic voltammetry.

Representative steady-state CVs are shown in Figure 1. The

potential window of the Cu(111) electrode in the pure

supporting electrolyte (10 mM HCl) is limited by the oxidative

copper dissolution reaction (CDR) at the anodic limit and the

Beilstein J. Org. Chem. 2014, 10, 2243–2254.

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reductive hydrogen evolution reaction (HER) at the cathodic

limit. At intermediate potentials, two peaks are seen which are

due to chloride desorption/adsorption at −360 mV and −80 mV,

respectively. Compared to this CV in pure hydrochloric acid,

drastic changes are found in the cyclic voltamogram of the

Cu(111) electrode in contact with the electrolyte containing

0.1 mM DBV2+.

Figure 1: Cyclic voltammograms of Cu(111) in pure 10 mM HCl(dashed grey curve) and in viologen molecules containing (10mM HCl+ 0.1 mM DBV2+) solution (solid black curve); dE/dt = 10 mV/s; E =electrode potenial. CDR = copper dissolution reaction, HER =hydrogen evolution reaction. Reproduced with the permission from[16].

The first difference relates to a considerable shift of the HER

towards lower potentials in the presence of the organic over-

layer. This shift is most likely caused by viologen molecules

blocking the most reactive surface sites for this reaction. The

same effect was also reported by Pham et al. [6] and

Safarowsky et al. [7] using a Cu(100) crystal as working elec-

trode.

The second most obvious deviation concerns the appearance of

three new pairs of peaks at potentials close to the HER. These

additional current waves, namely P1/P1’ and P2/P2’ and P3/P3’,

are assigned to viologen-related redox-processes (P1/P1’ and

P3/P3’) [5-7,13-16], as well as to an order/disorder phase tran-

sition due to chloride desorption/adsorption [17]. As mentioned

in the Introduction, the viologen dication (DBV2+) is known to

undergo two successive one-electron transfer steps in the elec-

trochemical environment forming first the viologen monoca-

tion radical (DBV+•) (Figure 2) and then the uncharged mole-

cule (DBV0) (for details see [5,6,13-16], and the papers cited

therein). While the dication and monocation radicals are soluble

in aqueous solutions, the uncharged molecules can accumulate

at the electrode surface due to their hydrophobic properties [4].

Figure 2: Reversible redox-state of viologen molecule; Φ = dihedralangle of the respective bipyridinium core.

The actual shape of the black cyclic voltammogram in Figure 1,

in particular the relative intensities of the various peaks can be

understood when considering the involvement of “solution

species” and “surface limited” reactions, respectively. Starting

at positive potentials, i.e., above P1/P1', the reduction/re-oxi-

dation of both, the limited number of pre-adsorbed viologen

dications (see XPS evidence below) and the continuously

arriving viologen cations from solution, can be described by

DBV2+ + e− DBV+•

While DBV+• species leaving the surface are known to form

dimers in solution [5], DBV+• species staying on the surface

may also form polymeric chains (see below):

2 DBV+• [DBV2]2+ (solution)

n DBV+• [DBVn]n+ (surface)

The reduction of the surface-confined species may even occur

at an electrode potential different form that for the reduction of

“solution species”.

The further reduction of the monocation radicals to the fully

uncharged viologen molecule DBV0 (peak P3 in Figure 1)

n DBV+•+ n e− n DBV0

occurs already within the regime of massive hydrogen evolu-

tion. Since under these conditions reliable in situ STM measure-

ments are not possible, the influence of this second reduction

step on the structure of the deposit is not considered further

here.

In the following sections we will now present and discuss in

situ STM images as obtained for the electrode surface in

different potential regimes. We start at potentials where the

molecules retain their dicationic character (DBV2+) in the

adsorbed state, and then continue with results taken at poten-

tials where the adsorbed molecules have undergone the first

one-electron reduction step (at P1 in Figure 1) and exist in their

Beilstein J. Org. Chem. 2014, 10, 2243–2254.

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monocation radical form DBV+•. These images clearly show the

decisive influence of the respective charge state of the molec-

ular species on the structure of the adsorbed DBV layer.

Structural characterizationsAs documented in the literature [18-22] a well-ordered

c(p × √3) layer of adsorbed chloride anions is formed on the

Cu(111) surface (Figure 3) in the supporting HCl electrolyte,

which, starting from a hexagonal c(√3 × √3)R30° structure at

negative potentials exhibits the phenomenon of reversible elec-

trocompression into a uniaxially incommensurate c(p × √3)

structure with rectangular unit cell at positive potentials. This

c(p × √3)Cl structure remains stable in the potential range

between the copper dissolution reaction and about −300 mV

(see Figure 1). In contrast to the larger halides, e.g., bromide

and iodide, the chloride anions retain to a large extent their

negative charge upon adsorption [23,24]. Hence, this regular

array of anions can be regarded, similar to the c(2 × 2)Cl layer

on Cu(100) (see below), as a suitable template for the adsorp-

tion of positively charged organic molecules.

Figure 3: Chloride-modified Cu(111) surface : a) STM image70 nm × 70 nm, bias voltage Ub +220 mV, tunneling current It = 0.2 nA,E = 50 mV; b) STM image of the c(p × √3)Cl structure: 14.4 nm ×14.4 nm, 5.58 nm × 5.58 nm, It = 2.0 nA, Ub = 75 mV, E = 0.0 mV;c) Hard-sphere model of the chloride-modified Cu(111) surface.

Exposing the c(p × √3)Cl terminated Cu(111) surface to the

electrolyte containing DBV2+ ions at potentials between

−50 mV and +50 mV vs RHE, i.e., in the potential regime

above the first reduction peak P1 in Figure 1, results in the

instantaneous formation of a highly ordered DBV2+ film.

Figure 4 shows representative STM images describing the

surface morphology and molecular structure of the

DBV2+adlayer. First, the straight step-edges in Figure 4a

running by each other at a typical angle of 120° still remain,

providing a first indication of the persistence of the c(p × √3)Cl

layer underneath the organic molecules (see also Figure 6). This

indicates that the DBV2+ adlayer has no significant impact on

the substrate-surface morphology, which is governed by the

chemisorptive Cu–Cl bond. Two distinguishable domains

rotated by an angle of 120° with respect to each other, denoted

as I/I’ and II/II’ on the two different terraces shown, are

observed in Figure 4a. A close inspection of the molecular

arrangement makes it clear that the DBV2+ molecular rows

within the domains are oriented parallel to step directions, the

latter being aligned along the close-packed anion rows within

the c(p × √3) chloride structure underneath [17,25]. As a result,

the DBV2+ molecular rows are oriented parallel to the commen-

surate direction of the chloride lattice. Alternatively, they are

aligned parallel to the directions of the Cu(111) substrate

(see Figure 3).

On the molecular level the rows consist of units of two bright

oval dots assigned to the bipyridinium cores of individual

DBV2+ molecules (Figure 2) that meet each other by an angle

of 120 ± 2°. Using a line profile measurement along the white

line in Figure 4c gives a length of about 0.72 ± 0.01 nm for one

of the units (Figure 4d). This value is in complete agreement

with the N–N distance of 0.71 nm within the DBV2+ molecules

[5,12]. Based on this agreement in size, the given angle of 120°,

and in particular, the consideration of electrostatic interactions

between the bipyridinium core and the benzyl groups (for more

details see the discussion below) we propose the molecular

arrangement as shown in Figure 4c.

The structural correlation between the DBV2+ adlayer and the

underlying chloride lattice could also be obtained by carefully

varying the tunneling conditions [5,7,26]. Under “soft tunneling

conditions”, i.e., with high bias voltage and low tunneling

current, the characteristic features of the DBV2+ adlayer are

observed (Figure 5a). Conversely, the chloride lattice under-

neath becomes visible when “drastic tunneling conditions” are

applied, i.e., low bias voltage and high tunneling current. In this

circumstance, the tunneling tip serves as a molecular brush to

locally remove the DBV2+ overlayer, leaving the c(p × √3)-Cl

lattice behind (Figure 5b). By comparing panels 5a and 5b, it

becomes evident that the individual bipyridinium cores of the

DBV2+ molecules are aligned parallel to the underlying close-

packed chloride rows, i.e., parallel to the directions of the

Cu(111) substrate, indicating the orienting effect of the lattice

of the specifically adsorbed chloride anions on the structure of

the adsorbed DBV2+ overlayer. This hints to a template effect

rather than a mere self-assembly of the molecular dications on

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Figure 4: Typical STM images of the surface morphology and high-resolution images of the DBV2+ related herring-bone phase on Cl/Cu(111):a) Surface morphology of the surface with a characteristic substrate step angle of 120° and two existing rotational domains (I/II; I'/II') of the DBV2+

herring-bone phase: 46.67 nm × 46.67 nm, Ub = 141 mV, It = 0.3 nA, E = −180 mV; b,c) Medium-scale (20.73 nm × 20.73 nm) and high-resolution(2.8 nm × 2.8 nm) STM image of the herring-bone phase, the latter showing two individual DBV2+ molecules in each structural element: Ub = 386 mV,It = 0.1 nA, E = +10 mV; d) Line profile recorded along the white line in Figure 4c indicating the length of the dipyridinium group of about 0.72 nm inperfect agreement with the N–N distance within the DBV2+ molecule. Figure 4a is reproduced with permission from [16].

Figure 5: Structural correlation between the ordered DBV2+ herring-bone phase and the anionic chloride lattice underneath: a) 6.5 nm × 4.1 nm,Ub = 220 mV, It = 0.1 nA, E = −10 mV; b) 6.5 nm × 4.1 nm, Ub = 30 mV, It = 5.0 nA, E = −10 mV. and are the unit vectors of the chlo-ride lattice and the herring-bone phase, respectively.

the Cl/Cu(111) surface. Again, based on a superposition of

panels 5a and 5b, a precise determination of the DBV2+ unit cell

with respect to the c(p × √3)-Cl phase underneath, the latter

serving as an internal calibration lattice, is possible. As a result,

the unit cell containing four DBV2+ molecules can be described

by a rectangular (2 × 2p) mesh with respect to the c(p × √3)-Cl

lattice (Figure 5a). The lattice constants are estimated to =

0.83 nm and = 3.32 nm, respectively, enclosing an angle of

88 ± 2°. Alternatively, the unit-cell of the DBV2+ adlayer can

be directly related to the copper substrate (1 × 1) mesh

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Figure 6: A series of STM images recorded with the same tunneling parameters (46.67 nm × 46.67 nm, Ub = 386 mV, It = 0.1 nA) showing the struc-tural phase transition from the herring-bone phase to an alternating stacked stripe phase. Disintegration of the herring-bone phase and growth of thestripes starts preferentially at defect sites and step edges: a) E = −220 mV; b) E = −260 mV; c) E = −270 mV; d) E = −285 mV.

assuming a (4p × 2√3) coincidence mesh. The surface coverage

per domain was also calculated as Θ = 0.25 ML with respect to

the underlying chloride lattice, or 14.49·1013 molecules/cm2. A

more detailed discussion of the molecular arrangement within

the herringbone rows is postponed until section Discussion

where we will also make a comparison with the corresponding

system DBV on Cl/Cu(100).

The DBV-dication based herring-bone structure remains stable

in the potential range more positive than −240 mV vs RHE, but

it decays below this potential, giving rise to a surface phase

transition. Namely, the herring-bone phase disintegrates gradu-

ally when the electrode potential approaches peak P1 (cathodic

potential sweep) in the solid black CV in Figure 1, where the

viologen dication species (DBV2+) are reduced to the corres-

ponding monocation radicals (DBV+•). Figure 6 shows this

decay of the herring-bone structure and the simultaneous

growth of the new stripe phase (Figure 6b and c) within the

potential regime from E = −240 mV to E = −285 mV, in which

the 120° step edge serves as a positional marker. The phase-

transition process starts preferentially at point defects and

domain boundaries (as marked by the white arrows in

Figure 6b) because this requires a relatively low activation

energy. Finally, the new stripe pattern is completed right after

the potential reaches the value of E = −285 mV (Figure 6d). The

observation of two rotational domains I and II (and I’ on the

lower terrace, respectively) rotated by 120° elucidates the influ-

ence of the underlying substrate on the adsorption not only of

the viologen dication (see above) but also of the monocation

radical species. In fact, taking the symmetry of the substrate

into account, three rotational domains in total should coexist on

the Cu(111) surface. Additionally, the characteristic angle of

120° between substrate steps remains unaffected by this phase-

transition process and rules out chloride desorption at these

potentials. This rather hints to the molecular reduction as being

the origin of the phase transition.

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Figure 7: Series of STM images showing the desintegration of the stripe pattern and restoration of the corresponding herring-bone phase uponsweeping back toward positive potentials, 46.67 nm × 46.67 nm, It = 0.1 nA; a) Ub = 240 mV, E = −300 mV; b) Ub = 200 mV, E = −240 mV;b) Ub = 233 mV, E = −165 mV.

The first electron transfer-induced phase transition from the

DBV2+ herring-bone phase to the stripe phase of the DBV+•

monocation radicals is a quasi-reversible process. The DBV2+

related herring-bone phase is gradually restored when the

working potential is swept back towards the positive potential

regime. A series of STM images recorded on the same surface

area but at increasing electrode potentials, showing a phase

transition from the stripe pattern back to the herring-bone phase,

is shown in Figure 7. The stripe phase, as observed in Figure 7a

(see inset), gradually desintegrates resulting in the reappear-

ance of the herring-bone phase (Figure 7b), which finally

completely replaces the stripe phase, as seen in Figure 7c (see

inset). Similar to the transformation from the DBV2+ herring-

bone phase to the DBV+• stripe pattern, step-edges and defect

points act as starting points for this phase-transition process.

Comparing panels 7a and 7c, it becomes evident that the molec-

ular row directions in the DBV2+ herring-bone phase are the

same as in the DBV+• stripe pattern, i.e., aligned parallel to sub-

strate step edges. This observation again affirms the dominant

role of interactions between the molecular adlayer and the

underlying chloride lattice for the lateral ordering.

Recalling the stripe phase of the DBV+• molecules, Figure 8

presents typical meso- and molecular-scale STM images of this

phase formed on the chloride-terminated Cu(111) electrode

surface. As mentioned above and shown in Figure 8a the stripes

are aligned parallel to the directions of step edges; these direc-

tions coincide with the close-packed chloride rows underneath,

and, hence, the directions of the Cu(111) substrate (see

[14] and papers cited therein). The higher resolution STM

image in Figure 8b reveals further details of the internal struc-

ture of the rows, i.e., the molecular orientation and packing

arrangement. The elongated and parallel discs within the rows

are assigned to individual DBV+• monocation radicals. Within

one row all molecules have the same orientation, whereas the

orientation of the monocation radicals in adjacent rows is alter-

nating, in that the molecules in neighboring rows are rotated by

120° with respect to each other (redish discs in Figure 8b)

leading to a zig-zag appearance. Hitherto this stripe pattern will

therefore be termed "alternating stripe" pattern, in contrast to

the findings on Cl/Cu(100) (see below). As a consequence not

only the directions of the rows are aligned to a symmetry direc-

tion of the substrate surface, but also the bipyridinium cores are

oriented in the direction of close-packed chloride rows under-

neath. Moreover, an even closer look reveals i) that within one

row all molecules are imaged with the same intensity,

suggesting equivalent adsorption sites, while ii) every second

row of the "alternating stripe" structure appears slightly brighter

in the STM image. Considering the uniaxial incommensuracy of

the chloride structure underneath in the direction perpendicular

to the rows (see Figure 3), the most likely explanation for the

latter phenomenon is that molecules in adjacent rows are situ-

ated on non-equivalent chloride rows underneath. The intermol-

ecular distances within one and the same row as well as

between adjacent stripes are measured as das= 0.43 ± 0.01 nm

and sas= 1.3 ± 0.1 nm, respectively [16].

Applying different tunneling conditions, as mentioned above,

the structural correlation between the DBV+• adlayer and the

underlying chloride lattice can again be made visible (not

shown here). On the basis of a superposition of both lattices, the

derived unit cell containing two DBV+• molecules can be

expressed by a (1 × 4) coincidence mesh with respect to the

c(p × √3)Cl lattice with the lattice constants of =

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Figure 8: a) Typical STM image showing two rotational domains of the DBV+• alterating stripe pattern (see text): 35.49 nm × 35.49 nm, Ub = 386 mV,It = 0.1 nA, Ew = −280 mV; b) Molecularly resolved STM image of the DBV+• alterating stripe pattern, 5.06 nm × 5.06 nm, Ub = 298 mV, It = 0.1 nA,Ew = −286 mV. Reproduced with permission from [16].

Figure 9: STM images of the DBV2+ cavitand phase on c(2 × 2)Cl/Cu(100); a) 29.2 nm × 29.2 nm, b) and c) 7.5 nm × 7.5 nm, It = 0.35 nA,Ub = 120 mV. Reproduced with permission from [7]. Copyright 2004 American Chemical Society.

0.43 ± 0.01 nm and = 3.32 ± 0.1 nm, respectively. From this

a DBV+•surface coverage per domain is calculated as

Θ = 0.25 ML with respect to the c(p × √3)Cl layer serving as

the template, or 14.30·1013 molecules/cm2.

DiscussionThe principles of structure formation of the adsorbed DBV

species become particularly clear when comparing the findings

for the ordered phases of DBV on Cl/Cu(111) with those

obtained on Cl/Cu(100). To this end we summarize here very

briefly the previously published results for the DBV adsorption

on Cl/Cu(100) in 10 mM HCl solution [5-7]. First of all the

chloride anions are known to form a well-ordered c(2 × 2)

structure on the Cu(100) surface (see Figure 11) between

−300 mV (near the HER) and the onset of the CDR around

+150 mV vs RHE. As illustrated in Figure 9 a highly ordered

layer of DBV2+ is observed at +50 mV on the chloride covered

Cu(100) electrode in contact with the 0.1 mM DBVCl2

containing 10 mM HCl electrolyte, forming a so-called “cavi-

tand”-structure consisting of small squares with a hole in the

center. Figure 9a shows two mirror domains of this phase

denoted as I and II enclosing an angle of 32° between them. Of

course, due to the four-fold symmetry of the Cu(100) surface

there exist also mirror domains I' and II' rotated by 90° resulting

in four possible domains in total.

A close inspection of the domain boundary in Figure 9a reveals

that occasionally the small squares are incomplete (white arrow

in Figure 9a) suggesting that the cavitands are actually formed

from subunits. This is verified by the two high-resolution zoom-

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Figure 10: Typical STM image showing two rotational domains of the DBV+• stripe pattern on Cl/Cu(100): a) 57.6 nm × 57.6 nm, Ub = 268 mV,It = 0.2 nA, E = −380 mV. Structural correlation between the stripe pattern and the underlying chloride lattice: 6.8 nm × 6.8 nm; b) Ub = 28 mV,It = 4 nA, E = −200 mV; c) Ub = 1 mV, It = 9 nA, E = −130 mV. Figure 10b and c are reproduced with permission from [5]. Copyright 2006 RoyalSociety of Chemistry.

ins in Figure 9b and c. Each cavitand consists of four indi-

vidual DBV2+ species building a square-shaped motif with a

cavity in the center. Since the four building blocks may be

arranged in two different ways as illustrated in the two zoom-

ins these cavitands occur in two circularly chiral enantiomers

[5-7]. However, since neither the Cl/Cu(100) surface nor the

DBV2+ species (Figure 2) are chiral in nature the DBV2+

covered Cl/Cu(100) surface as a whole is a racemate of enan-

tiomeric domains like I and II in Figure 9a. The dicationic char-

acter of the DBV2+ building blocks of this cavitand structure

was verified by ex situ XPS measurements using synchrotron

radiation, namely by a dominant N(1s) signal at 402.1 eV [27].

Sweeping the electrode potential to a value below −200 mV vs

RHE causes the disintegration of the cavitand structure and the

formation of a stripe pattern (Figure 10) similar to the one

shown in Figure 7a (inset) except that here the orientation of the

individual molecular species in adjacent stripes is the same and

not alternating. This new phase is a consequence of the reduc-

tion of the adsorbed DBV2+ dications (i.e., the building blocks

of the cavitand structure) to monocation radicals DBV+• [5,6],

which form the polymeric stripes. The distance between adja-

cent stripes is ss = 1.8 nm and the intermolecular distance

within the DBV+• stripes is ds = 3.6 Å. A correlation of the

molecular structure of the stripes with that of the Cl-lattice

underneath as deduced from STM images taken at different

tunneling conditions as described above (Figure 10b,c) proves

that the bipyridinium cores of the DBV+•species are again

oriented parallel to the close-packed chloride rows underneath.

Unlike on Cl/Cu(111), however, not only have molecules in

adjacent rows the same orientation but all molecular stripes on

the Cl/Cu(100) appear also with the same brightness

(Figure 10b).

All observations made for the adsorption of DBV on the chlo-

ride precovered Cu(111) and Cu(100) electrode surfaces for

both the dicationic DBV2+ and the monocation radical DBV+•

species, respectively, are summarized in the structural models

of Figure 11a–d.

First of all in both the DBV2+dication and the DBV+•monoca-

tion radical species the positive charge resides on the

N-containing bipyridinium core as revealed by the N(1s)

photoemission spectra [27], while the two benzyl groups,

decoupled from the delocalized π-system of the bipyridinium

core by the two CH2 groups, are relatively more negatively

charged. As a consequence the preferred adsorbate–adsorbate

interactions are electrostatic attractions between the positive

bipyridinium cores and the benzyl groups. In addition π–π-inter-

actions between all π-systems of adjacent molecules will play a

role with the stipulation that the benzyl groups take up a trans-

conformation [6] as shown in Figure 2 and Figure 8b and in the

structural models in Figure 11. In particular, the π–π interac-

tions between neighboring monocation radicals are important

which, by spin-pairing, are known to lead to the formation of

dimers in solution [5]. The adsorbate–substrate interactions are

obviously dominated by electrostatic interactions between the

doubly (DBV2+) or singly (DBV+•) charged bipyridinium

cations and the negatively charged chloride layer underneath,

with the additional remark that the anion density, and thus the

negative charge density, on the chloride precovered Cu(111)

surface is higher than on Cl/Cu(100). This manifests itself in the

fact, that the respective coverages are consistently higher on the

Cl/Cu(111) surface compared to Cl/Cu(100), namely Θ(DBV2+)

= 0.075 and Θ(DBV+•) = 0.20 on Cl/Cu(100) vs Θ(DBV2+) =

0.25 and Θ(DBV+•) = 0.25 on Cl/Cu(111).

Beilstein J. Org. Chem. 2014, 10, 2243–2254.

2252

Figure 11: Structure models for the observed ordered layers of DBV2+ dications and DBV+• monocation radicals on a c(p × √3)Cl/Cu(111) and ac(2 × 2)Cl/Cu(100) electrode surface, respectively, as derived from in situ STM measurements.

Based on these possible interactions all structures of both the

DBV2+ dications and the DBV+• monocation radicals are

consistently explainable (Figure 11). In all cases the positively

charged bipyridinium cores are oriented parallel to the close-

packed rows of the chloride anion lattice underneath in order to

maximize the electrostatic attraction. This explains the parallel

and 90° vs parallel and 120° orientation of both the individual

DBV2+ or DBV+• bipyridinium moieties as well as corres-

ponding structural domains with respect to each other, on the

Cl/Cu(100) and Cl/Cu(111) surface, respectively. Electrostatic

repulsion between parallel oriented DBV2+ units leads to rela-

tively large distances between them within the cavitand struc-

ture on Cu(100) and the “herring-bone” structure on Cu(111).

Interestingly, these distances are multiples of the nearest-

neighbor Cl–Cl distance, the shortest being 0.83 nm within the

“herring-bone” structure on Cl/Cu(111). The directions of mole-

cular rows of both the "alternating stripe" structure of

DBV2+dications and the stripe phase of DBV+• monocation

radicals on Cl/Cu(111) are also aligned with the directions of

close-packed chloride ions underneath, resulting in three

possible domains in total rotated by 120° with respect to each

other. The existence of the mirror domains of the cavitand

structure ±16° off the direction of the two orthogonal directions

of densely packed chloride rows on Cu(100) (Figure 11a), in

turn, leads to four possible domains of the DBV2+cavitand

structure on Cl/Cu(100). Likewise, the molecular rows of the

DBV+• stripe phase on Cl/Cu(100) propagate 37° off the direc-

tion of closely packed chloride anions. This, together with the

twofold symmetry of the Cu(100) substrate, results again in a

total of four possible domains of this structure on Cl/Cu(100).

Summarizing so far, the occurrence of the angles of 90° and

120° and the coincidence of the connecting line between the

two N-atoms of a bipyridinium core with the direction of close-

packed anion rows of c(p × √3)Cl/Cu(111) and c(2 × 2)Cl/

Cu(100) reflects the influence of the symmetry of the respec-

tive substrate and the dominance of electrostatic adsorbate–sub-

strate interactions.

Looking into the fine structure of the various phases, we find

the largest distance as well as an orthogonal orientation between

the DBV2+ as a consequence of the electrostatic repulsion

between these dications in the cavitand phase on Cl/Cu(100).

The benzyl groups inside the cavity may partly shield this repul-

sion between the DBV2+ units (Figure 11a). Within the DBV+•

Beilstein J. Org. Chem. 2014, 10, 2243–2254.

2253

stripes on Cl/Cu(100) the distance between the molecular units

is only 0.36 nm due to a reduced electrostatic repulsion and an

attractive π–π- and spin-pairing interaction between the parallel

oriented monocation radicals (Figure 11b). Similar intermolec-

ular distances have also been observed for π–π stacked phases

of 2,2'-bipyridine on Au(111) [28] and Cu(111) [26]. Even

though this distance is typical for π–π-interacting aromats [29],

the measured distance of 0.36 nm agrees perfectly with the sep-

aration between parallel densely packed rows of chloride

anions, again a manifestation of the strong electrostatic inter-

action with the substrate. The lateral displacement between

adjacent DBV+• bipyridinium cores within the rows, leading to

the mirror domains off by ±37° from the directions of the

substrate, is probably due to steric hindrance between the trans-

oriented benzyl groups of the DBV+• units.

Also on the Cl/Cu(111) substrate the intermolecular distance of

the parallel DBV2+ dications within the rows is large, and with

0.83 nm nearly twice the Cl–Cl distance along the commensu-

rate direction of the c(p × √3)Cl layer between closely packed

rows of the anion underlayer. This distance leaves enough space

between two positively charged bipyridinium cores for a benzyl

group of the neighboring row, thereby shielding the electro-

static repulsion between the former (Figure 11c). As a further

consequence, the trans-conformation of the DBV2+ species

matches the 120° orientation of adjacent bipyridinium cores

within the “herring-bone” structure. The remaining benzyl

groups may also π–π interact and are located within the dark

lines between the “herring-bones”. The model in Figure 11d

summarizes the "alternating stripe" structure formed by the

DBV+• species on Cl/Cu(111). Both the individual monocation

radicals as well as the stripe propagation direction are aligned in

directions, molecules in adjacent rows being rotated by

120°. The intermolecular distance of 0.43 nm within the rows,

though still consistent with π–π interaction, equals precisely the

Cl–Cl distance in the direction of the substrate, again

pointing to a dominance of adsorbate–substrate interactions.

ConclusionThe electrochemical behavior and a related structural transition

of the 1,1’-dibenzyl-4,4’-bipyridinium molecule cations on a

chloride-modified Cu(111) surface have been investigated by

means of cyclic voltammetry and in situ scanning tunneling

microscopy. Current waves in the cyclic voltammogram clearly

indicate the potentials where reduction of the dications occurs

first to the monocation radicals and then to the neutral mole-

cules. At positive electrode potentials the dicationic DBV2+

molecules form a condensed and highly ordered "herring-bone"

phase consisting of structural elements each formed by two

individual DBV2+ molecules. In contrast, an "alternating stripe"

pattern is observed for the molecules in their monocation

radical form (DBV+•) at negative potentials below the first

reduction peak. In both cases, their structural motifs are

predominantly governed by dominant electrostatic interactions

between the adsorbate species, both in their dication and mono-

cation radical form, and the negatively charged chloride lattice

underneath. The phase transition from the DBV2+-related

"herring-bone" phase to the alterating stripe pattern based on the

radical monocationic DBV+• is observed as a reversible process

occurring via nucleation and growth. Possible models for both

the herring-bone phase and the alternating stripe pattern are

proposed. Their detailed discussion also in the light of the

corresponding findings for the same species on a c(2 × 2)Cl/

Cu(100) electrode surface clearly points to a dominance of elec-

trostatic adsorbate–substrate interactions, i.e., a strong template

effect of both substrates on the self-organization of these

organic surface films.

ExperimentalIn this work we have employed a combination of cyclic voltam-

metry (CV) and electrochemical scanning tunneling microscopy

(EC-STM). The direct combination of in situ STM and CV in

one and the same electrochemical cell permits a precise correla-

tion of the obtained STM images with features in the corres-

ponding CV data. The whole experimental setup is home-built

and described in detail in [25]. The tunneling tips used in all

experiments were electrochemically etched from 0.25 mm in

diameter tungsten wire in 2 mM KOH solution, rinsed with high

purity water, dried and subsequently insulated by passing them

through a hot melt glue film (ethylen–vinylacetat copolymer).

The Cu(111) single crystal used was manufactured by MaTeck,

Jülich, Germany. Prior to each series of STM measurements the

copper sample was electropolished by immersing it into 50%

orthophosphoric acid at an anodic potential of 2 V for about

20–40 s. This removes the native surface oxide film formed in

air. In order to guarantee a reproducibly smooth surface even

after several electropolishing procedures, a precision of the

surface orientation of less than 0.5° off the (111) plane is

required. Less well defined surfaces suffer a growing rough-

ening with repeated cycles of electropolishing.

The chloride-modified Cu(111) surface, chosen here as sub-

strate for the viologen films, was prepared and characterized by

first carrying out CV and STM measurements several times in

pure 10 mM HCl solution, i.e., the supporting electrolyte. This

procedure also improves the quality of the Cu(111) surface due

to the operation of the so-called “electrochemical annealing

effect” [17,30]. For the adsorption of the molecular film on top

of the chloride-terminated electrode surface, the pure supporting

electrolyte was routinely substituted by a solution of 0.1 mM

1,1’-dibenzyl-4,4’-bipyridinium dichloride in 10 mM HCl

Beilstein J. Org. Chem. 2014, 10, 2243–2254.

2254

(10 mM HCl + 0.1 mM DBV2+) at potentials between −50 mV

and +50 mV vs RHE (reversible hydrogen electrode). In this

paper all potentials of the Cu(111) electrode, i.e., the working

electrode, are quoted with respect to a reversible hydrogen elec-

trode, while a Pt wire is employed as counter electrode.

High purity water from a Milli-Q purification system (conduc-

tivity > 18 MΩ·cm, TOC < 4 ppb) and highest reagent grade

chemicals were used for the preparation of all solutions. All

electrolyte solutions were purged with oxygen-free argon gas

for several hours before use.

AcknowledgementsThis work has been supported by the SFB 624 of the Deutsche

Forschungsgemeinschaft (DFG) foundation.

References1. Lehn, J.-M. Supramolecular Chemistry; Wiley-VCH: Weinheim,

Germany, 1995.2. Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89–112.

doi:10.1002/anie.1988008913. Vögtle, F. Supramolecular Chemistry: An Introduction; Wiley and Sons:

Chichester, UK, 1991.4. Kaifer, A. E.; Gómez-Kaifer, M. Supramolecular Electrochemistry;

Wiley-VCH: Weinheim, Germany, 1999. doi:10.1002/97835276136015. Pham, D.-T.; Gentz, K.; Zörlein, C.; Hai, N. T. M.; Tsay, S.-L.;

Kirchner, B.; Kossmann, S.; Wandelt, K.; Broekmann, P. New J. Chem.2006, 30, 1439–1451. doi:10.1039/b609421j

6. Pham, D.-T.; Tsay, S.-L.; Gentz, K.; Zörlein, C.; Kossmann, S.;Stay, J.-S.; Kirchner, B.; Wandelt, K.; Broekmann, P. J. Phys. Chem. C2007, 111, 16428–16436. doi:10.1021/jp073469q

7. Safarowsky, C.; Wandelt, K.; Broekmann, P. Langmuir 2004, 20,8261–8269. doi:10.1021/la048940a

8. Haiss, W.; van Zalinge, H.; Higgins, S. J.; Bethell, D.; Höbenreich, H.;Schiffrin, D. J.; Nichols, R. J. J. Am. Chem. Soc. 2003, 125,15294–15295. doi:10.1021/ja038214e

9. Li, Z.; Han, B.; Meszaros, G.; Pobelov, I.; Wandlowski, T.;Blaszczyk, A.; Mayor, M. Faraday Discuss. 2006, 131, 121–143.doi:10.1039/b506623a

10. Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.;Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100–110.doi:10.1021/ja002154k

11. Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49–82.doi:10.1039/cs9811000049

12. Monk, P. M. S. The Viologens: physicochemical properties, synthesisand applications of the salts of 4,4’-bipyridines; John Wiley and SonsLtd.: Chichester, UK, 1998.

13. Tsay, S.-L.; Tsay, J.-S.; Fu, T.-Y.; Broekmann, P.; Sagara, T.;Wandelt, K. Phys. Chem. Chem. Phys. 2010, 12, 14950–14959.doi:10.1039/c0cp00865f

14. Röefzaad, M.; Jiang, M.; Zamlynny, V.; Wandelt, K.J. Electroanal. Chem. 2011, 662, 219–228.doi:10.1016/j.jelechem.2011.07.006

15. Kobayashi, K.; Fujisaki, F.; Yoshimine, T.; Nik, K. Bull. Chem. Soc. Jpn.1986, 59, 3715–3722. doi:10.1246/bcsj.59.3715

16. Phan, T. H.; Wandelt, K. Int. J. Mol. Sci. 2013, 14, 4498–4524.doi:10.3390/ijms14034498

17. Broekmann, P.; Wilms, M.; Kruft, M.; Stuhlmann, C.; Wandelt, K.J. Electroanal. Chem. 1999, 467, 307–324.doi:10.1016/S0022-0728(99)00048-0

18. Stickney, J. L.; Ehlers, C. B. J. Vac. Sci. Technol., A 1989, 7,1801–1805. doi:10.1116/1.576049

19. Kruft, M.; Wohlmann, B.; Stuhlmann, C.; Wandelt, K. Surf. Sci. 1997,377–379, 601–604. doi:10.1016/S0039-6028(96)01461-6

20. Wilms, M.; Broekmann, P.; Kruft, M.; Stuhlmann, C.; Wandelt, K.Appl. Phys. A 1998, 66 (Suppl. 1), S473–S475.doi:10.1007/s003390051185

21. Phan, T. H.; Wandelt, K. Surf. Sci. 2013, 607, 82–91.doi:10.1016/j.susc.2012.08.013

22. Phan, T. H. Ph.D. Thesis, Bonn University, Bonn, Germany, 2012.23. Batina, N.; Kunitake, M.; Itaya, K. J. Electroanal. Chem. 1996, 405,

245–250. doi:10.1016/0022-0728(95)04480-924. Koper, M. T. M. J. Electroanal. Chem. 1998, 450, 189–201.

doi:10.1016/S0022-0728(97)00648-725. Wilms, M.; Kruft, M.; Bermes, G.; Wandelt, K. Rev. Sci. Instrum. 1999,

70, 3641–3650. doi:10.1063/1.114997126. Safarowsky, C.; Rang, A.; Schalley, C. A.; Wandelt, K.; Broekmann, P.

Electrochim. Acta 2005, 50, 4257–4268.doi:10.1016/j.electacta.2005.03.068

27. Breuer, S.; Pham, D. T.; Huemann, S.; Gentz, K.; Zoerlein, C.;Hunger, R.; Wandelt, K.; Broekmann, P. New J. Phys. 2008, 10,125033. doi:10.1088/1367-2630/10/12/125033

28. Dretschkow, T.; Wandlowski, T. Electrochim. Acta 1999, 45, 731–740.doi:10.1016/S0013-4686(99)00252-2

29. Weck, M.; Dunn, A. R.; Matsumoto, K.; Coates, G. W.;Lobkovsky, E. B.; Grubbs, R. H. Angew. Chem., Int. Ed. 1999, 38,2741–2745.doi:10.1002/(SICI)1521-3773(19990917)38:18<2741::AID-ANIE2741>3.0.CO;2-1

30. Giesen, M.; Beltramo, G.; Dieluweit, S.; Müller, J.; Ibach, H.;Schmickler, W. Surf. Sci. 2005, 595, 127–137.doi:10.1016/j.susc.2005.07.040

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